Portable sample cylinders for collecting and transporting fluid samples are generally of the spun-end sample body variety or the piston-type sample cylinder variety. When sampling certain fluids, such as ethane, propane, butane, or other liquified petroleum gases (LPG), accurate sampling techniques require that the physical state of the fluid not change from the container (e.g., the LPG line) to the portable sample cylinder. Piston-type sample cylinders are thus generally preferred for sampling these fluids, since the sample can be input to the cylinder or vessel and maintained for subsequent analysis at the same pressure as the main container or fluid line. Moreover, a piston-type sample cylinder does not require that the sample be flushed through the cylinder, and thus only the precharged inert gas is released to either the atmosphere or a separate holding vessel.
The concentration of certain sample constituents within a portion of the sample cylinder may vary over time, yet a homogeneous sample fluid is required to perform accurate analysis on the sampled fluid. Various techniques have thus been devised for mixing the fluid in the sample cylinder prior to releasing the fluid from the cylinder for analysis. According to the plunger technique as disclosed in U.S. Pat. No. 3,793,888, an axially movable plunger is provided in the sample chamber for mixing the sample. This technique, however, is not preferred due to safety considerations, since the plunger is fully extended from the sample cylinder housing when the sample cylinder is full, and thereby creates shipping difficulties. Moreover, many gas samples are maintained at pressures within the cylinder such that the sampled gas behaves substantially as an incompressible fluid. Axial movement of the plunger into the cylinder reduces the effective volume of the sampled gas, further increasing its pressure and reducing its additional compressibility. Accordingly, the plunger may be difficult to compress, and thus considerable time is required to mix the sample.
Another variety of mixer for a sample cylinder utilizes a mixing ball movable by gravity in the sample chamber. This type of sample cylinder, as disclosed in U.S. Pat. No. 3,789,670, may be turned end-to-end for moving the mixing element from an end cap to the piston and back to the end cap. Voids within the sample chamber are preferably minimal, and accordingly the end cap and piston may be provided with a semi-spherical recess to jointly accomodate the mixing ball. In addition to the considerable manufacturing expense associated with providing the recesses, the mixing ball itself may act as a check valve, thereby preventing the sample from being discharged from the sample cylinder. Moreover, spherical balls do not function well to mix fluid in a cylindrical chamber, and thus again considerable time is required to mix the sample.
The seals on the piston in the sample cylinder are critical to the objective of obtaining a good sample. The sampled fluid must be kept isolated from the inert gas in the sample cylinder, and the inert gas must be kept isolated from the sampled fluid. The piston should sealingly move within the sample chamber under only a slight pressure differental. The use of conventional lubricants might minimize these problems, but such lubricants also absorb certain chemical groups in LPG, natural gas, or other samples, and are thus not recommended.
Ceramic magnets have been positioned on the piston of prior sample cylinders for activating external magnetic trip flags to obtain an indication of the position of the piston relative to the sample cylinder housing, and thus an indication of the sample volume. The magnets frequently shatter when impacted, however, causing significant damage to the interior wall of the sample cylinder and/or the seals or the piston, and may result in the loss of the sample when shattered pieces destroy the sealing effectiveness of the piston seals.
The above-described magnetic trip flag technique does not, however, provide an accurate reading of the sample quantity within the sample cylinder. Individual trip flags may be partially rather than fully turned, and thus the true position of the piston may vary 1/4" or more from the measured position, depending upon the reader. Inaccurate reading of the axial position of the piston in the sample cylinder may lead to a failure in recognizing a problem in the sampling procedure. For instance, if the sampling system is established to retrieve 50 cc of gas per day from a flow line and the system is checked after 3 days operation, the sample cylinder should indicate that 150 cc of sampled gas are in the cylinder. If, however, the sample reading is only 130 cc, the operator may assume the error lies in the accuracy of the indicator and thus assume the sample procedure is proper. On the other hand, if the operator is confident of the accuracy of the sample cylinder reading and the 130 cc reading is obtained, the operator will more thoroughly check the system and may discover, for example, that 20 cc pumped from the line to the sample cylinder was inadvertently lost due to a loose hose connection. In this case, the problem can thus be easily cured and the sample procedure accurately reinitiated.
Another variety of piston-type sample cylinder utilizes an indicator rod attached to the piston to relate the position of the piston and thus the sample quantity. This type of sample cylinder, as disclosed in U.S. Pat. No. 4,409,850, suffers from many of the safety and shipping problems associated with the plunger-type sample cylinder, since the indicator rod extends from the sampling housing. Also, it is time consuming to record the precise axial position of the rod with respect to the cylinder at various stages of the sampling process because marking or gradations on the rod would adversely affect the necessary sealing reliability between the end plate of the sample cylinder and the protruding rod.
Due in part to safety problems presented by a piston-type sample cylinder with an indicator rod or a plunger, prior art piston-type sample cylinders are generally shipped in a special container or "gun case" designed to protect the sample housing, related valves and gauges, and any rods or plungers extending from the housing. The containers themselves represent a considerable expense to the overall cost of obtaining and shipping samples, but have heretofore been considered necessary to fully protect the shipped sample.
The disadvantages of the prior art are overcome by the present invention, and an improved portable piston-type sample cylinder or vessel is hereinafter provided.